DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

pheophytin A phytoplankton pigment that is
inert and does not contribute to photosynthesis.
phi unit A measure of sediment size commonly
used in geology. Sediment diameter and
φ-size are related by D= 2 −φ , so that a larger
particle has a smaller φ value.
Phobos One of the two moons of Mars, also
designated MI. Phobos (meaning fear) was discovered
by A. Hall in 1877. Its orbit has an
eccentricity of 0.015, an inclination of 1.0 ◦ ,a
precession of 158.8 ◦ yr −1 , and a semimajor axis
of 9378 km. It is one of the three satellites in
our solar system whose period (7h 39m) is less
than the rotational period of the primary planet
(24h 37m for Mars); rises in the west and sets
in the east, often twice a day. It is losing orbital
energy to the surface tides it raises on Mars; its
orbit is decaying at a rate of 1.8 m/century and
in about 4 × 10 7 yr it will either be disrupted
by tidal forces (in this way, it may become a
ring plane about Mars within the next 50 million
years) or it will crash into the surface of
Mars. Its size is 13.5×10.8×9.4 km, its mass is
1.08×10 16 kg, and its density is 1.9 g cm −3 . Its
geometric albedo is 0.06 and its surface is similar
in reflectivity to C-type asteroids. It may
be a member of that group that was captured
in the past. New temperature data and closeup
images of Phobos gathered by NASA’s Mars
Global Surveyor indicate the surface has been
pounded into powder, at least 1m(3 ft) thick,
by eons of meteoroid impacts. Meteorite impacts
also initiated landslides along the slopes of
some craters. High temperatures were measured
at−4 ◦ C (25 ◦ F) and lows at−112 ◦ C(−170 ◦ F).
(Phobos does not have an atmosphere to hold
heat in during the night.)
Phoebe Moon of Saturn, also designated
SIX. It was discovered by Pickering in 1898. Its
orbit has an eccentricity of 0.163, an inclination
of 177 ◦ , and a semimajor axis of 1.30×10 7 km.
Its radius is 110 km, its mass is 4.0× 10 18 kg,
and its density is 0.72 g cm −3 . Its geometric
albedo is 0.05, much lower than the other Saturnian
satellites (except the dark part of Iapetus).
It orbits Saturn once every 550.5 Earth
days. Phoebe’s retrograde orbit indicates that it
is probably a captured satellite.
© 2001 by CRC Press LLC
photon
photodissociation region A part of the interstellar
medium where the molecules have been
destroyed into atoms by ultraviolet light, and
the chemical composition and temperature are
determined by the intensity of the light. See interstellar
medium.
photoelectrons Electrons emitted by the action
of light, in particular (in magnetospheric
research) of short-wave sunlight impinging on
either the upper atmosphere or on the surface
of a satellite. Photoelectron emission from the
upper atmosphere is the major source of the
ionosphere. Photoelectron emission from satellite
surfaces causes spacecraft charging and can
affect the operation of sensitive instruments in
space. See also differential charging.
photoinhibition The decrease in photosynthetic
rate with increasing irradiance, caused by
surpassing the photosynthetic capacity.
photoionization Ionization of atoms and ions
by photons. A photon with energy greater than
a threshold energy, the ionization potential, is
absorbed, and an electron previously bound to
an atom is freed. The energy gained by the
electron is equal to the difference between the
photon energy hν0 and the ionization potential
ɛ0, according to the formula hν = hν0 − ɛ0.
Spectral lines are emitted following photoionization
when the electron recombines. In astronomy,
such photoionized nebulae include Hi
regions, planetary nebulae, extended envelopes
surrounding hot stars, and the line-emitting regions
of active galactic nuclei.
photometric binary An orbiting pair of stars,
neither of which eclipses the other, but whose
gravitational fields distort each others’ shapes
enough so that the surface area we see (hence
the brightness of the system) changes through
the orbit period. Also ellipsoidal variable or ellipsoidal
binary.
photometry The accurate measurement of
the flux of light from a source.
photon An elementary particle of zero mass
and spin one. The quantum of electromagnetic
radiation; the smallest “bundle of energy”
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